The present invention relates in general to magnetic field sensors. More specifically, the present invention relates to using magneto-resistive sensors as multi-purpose sensors.
There are many applications in which there is a need to measure a magnetic field. Among such applications are magnetic compassing, traffic detection, navigation systems, as well as medical, laboratory and electronic instruments, for instance.
Such applications frequently employ magnetoresistive (xe2x80x9cMRxe2x80x9d) sensors capable of sensing small magnetic fields and their perturbations. Magnetoresistive sensors are often formed using integrated circuit fabrication techniques and are composed of a nickel-iron (permalloy) thin film deposited on a silicon wafer, or other types of substrate, and patterned as resistive strips. When a current is applied to a magnetoresistive sensor, the resistance of the strip depends on the angle between the magnetization and the direction of the applied current, and is maximized when the magnetization and the applied current are parallel. If the permalloy film is subjected to an external magnetic field, the field influences the magnetization, rotating it and thereby changing the film""s resistance. Typically, the maximum change in resistance due to rotation of the magnetic field is two to three percent of the nominal resistance.
During manufacture, the easy axis (a preferred direction of magnetization) is set to one direction along the length of the film to allow the maximum change in resistance for an applied field within the permalloy film. However, the influence of a strong magnetic field along the easy axis could rotate the polarity of the film""s magnetization, thus, changing the sensor""s characteristics. Following such changes, a strong restoring magnetic field is typically applied to restore, or set, the sensor""s characteristics. In certain designs, large external magnets can be placed to reset the sensor""s settings. However, such an implementation may not be feasible when a magnetoresistive sensor has already been packaged into a system. Particularly, some applications require several sensors within a single package to be magnetized in opposite directions. In such applications, instead of using large external magnets, individual coils may be wrapped around each sensor to reset sensor""s characteristics. Alternatively, current straps, also known as set-reset straps and offset straps, may be used to restore the sensor""s characteristics. The use of current straps in a magnetic field sensing device is discussed in the U.S. Pat. No. 5,247,278 to Bharat B. Pant, assigned to the same assignee as the current application. U.S. Pat. No. 5,247,278 is fully incorporated herein by reference.
In addition to magnetoresistive sensors, giant magnetoresistive (xe2x80x9cGMRxe2x80x9d) sensors are often used in many applications that require measurements of a relatively small magnetic field. Unlike magnetoresistive sensors, GMR sensors are composed of a multi-layer film deposited on a substrate, and the magnetoresistance occurs as a result of a relative magnetization angle between two adjacent layers, and the current direction typically does not play any role. Thin-film GMR materials deposited on a silicon substrate, or any other substrate, can be configured as resistors, resistor pairs, half bridges or Wheatstone bridges. Unlike magnetoresistive sensors, GMR sensors often do not employ set-reset straps in their configurations.
Many electronic components, such as semiconductor devices or Liquid Crystal Displays (xe2x80x9cLCDxe2x80x9d), as well as consumer and recreation products such as a compass or global positioning system (xe2x80x9cGPSxe2x80x9d) products, can be damaged by exposure to high or low temperatures. Thus, when temperature limits are exceeded, such components have to be protected from breakdown or malfunction. In systems including temperature sensitive components, temperature sensors play a key role in maintaining the reliability of the system""s components.
A number of temperature sensing techniques are currently used, and the most commonly used temperature sensors include resistive temperature detectors (xe2x80x9cRTDsxe2x80x9d), thermocouples, and sensor integrated circuits (xe2x80x9cICsxe2x80x9d). Resistive temperature sensors employ a sensing element whose resistance varies with temperature. For example, a platinum resistive temperature detector consists of a platinum wire coil that is wound around a film of platinum deposited on a substrate. A thermocouple, on the other hand, consists of a two-wire junction made of two different metals. Finally, a silicon sensor is an integrated circuit that typically includes extensive signal processing circuitry within a package housing the sensor.
With the increasing usage of sensors, a growing number of consumer and commercial products requires a combination of temperature and magnetic sensors. Unfortunately, because of the size, cost and other constraints, a compromise often has to be made among the several desired sensors included in a product. Thus, there is an apparent need for low-cost, multi-purpose sensors.
The system and methods are illustrated for an integrated dual-purpose sensing device.
One embodiment of an integrated dual purpose sensing device includes at least a first magnetoresistive element and a second magnetoresistive element, where each magnetoresistive element has a first sensing terminal and a second sensing terminal. According to an exemplary embodiment, the second sensing terminal associated with the first magnetoresistive element is connected to a first sensing terminal associated with the second magnetoresistive element. The first sensing terminal associated with the first sensing element is further connected to a power source. According to an exemplary embodiment, the integrated dual purpose sensing device is adaptable to provide two output measurements that are used to determine a temperature sensor reading and a magnetic sensor reading.